In the field of compact communication devices typified by smartphones, communication traffic has increased steeply, and devices have many more functions. To cope with the increase in communication traffic, the number of bands has been increased, whereas compact communication devices have been required to have many more functions, without increase in size. For various members used in compact communication devices, accordingly, further reduction in size and higher performance are essential.
An oxide single crystal such as lithium tantalate (LT) and lithium niobate (LN) is a typical piezoelectric material and is widely used as a material of surface acoustic wave (SAW) devices. When used as a piezoelectric material, the oxide single crystal, which has a large electromechanical coupling factor indicating a conversion efficiency from electromagnetic energy to mechanical energy, can achieve a wider bandwidth. The oxide single crystal however has low temperature stability, and a temperature change shifts the operable frequency thereof. The low temperature stability arises from a thermal expansion coefficient of the oxide single crystal.
As a method of improving the temperature stability of an oxide single crystal used as an electrical voltage material, the following method is disclosed, for example: an oxide single crystal wafer is bonded to a material having a smaller thermal expansion coefficient than that of the oxide single crystal, specifically, to a sapphire wafer; and the laminate is shaved from the oxide single crystal wafer side into several micrometers to several tens of micrometers, thereby suppressing the effect of the thermal expansion of the oxide single crystal (Non-Patent Document 1). In the method, the oxide single crystal wafer after the lamination is shaved, and thus most of the oxide single crystal wafer is unfortunately discarded, resulting in poor material utilization efficiency. Lithium tantalate or lithium niobate used as the oxide single crystal is an expensive material, and thus to reduce production costs, there is a demand for a technique with high material utilization efficiency to discard small amounts of materials.
The technique of manufacturing an SOI wafer, for example, a Smart-Cut method, is simply a method in which a silicon wafer including a hydrogen ion layer is bonded to a support wafer, and the laminate is heated at about 500° C. to thermally release the ion-implanted layer (Patent Document 1). In order to improve the utilization efficiency of an oxide single crystal wafer in a product, the following method has been tried: an oxide single crystal wafer is used in place of the silicon wafer in the Smart-Cut method to form a thin film of the oxide single crystal on a support wafer (Non-Patent Documents 2 and 3).
Non-Patent Document 2 discloses the following technique: on the surface of a lithium tantalate wafer including an ion-implanted layer, a Cr metal layer having a thickness of 121 nm is formed; a SiO2 substrate having a thickness of several hundreds of nanometers is bonded through the metal layer; the laminate is heated at 200 to 500° C. to separate the laminate along the ion-implanted layer, thus transferring a lithium tantalate thin film onto the SiO2 substrate through the metal layer; and then a lithium tantalate wafer is bonded to the face of the SiO2 substrate opposite to the face to which the lithium tantalate thin film has been transferred, thereby manufacturing a lithium-tantalate-metal-on-insulator (LTMOI) structure. Non-Patent Document 3 discloses the following technique: a silicon wafer is bonded to a lithium tantalate wafer including an ion-implanted layer; and the laminate is heated at 200° C. to separate the laminate along the ion-implanted layer, thereby thermally transferring a lithium tantalate thin film onto the silicon wafer.